2 Answers
2

In the solar system, the planets follow orbits determined mainly by
the Sun's gravity --- since the Sun is the most massive object in
the system (it is about 1000 times as massive as Jupiter, which is
about 300 times more massive than Earth). If the Sun and Earth were
the only things in the solar system, our planet's orbit would be an
ellipse of virtually constant shape and orientation in space. However,
because the other planets pull on Earth, our orbit does change
slightly over time, in orientation and in shape. Same for the other
planets' orbits. The biggest effect is that the orbit of a planet
(which is nearly exaclty an ellipse) gradually shifts its orientation
over time: the shifting is like how a circular hula-hoop (representing
the orbit) shifts in location around the hula-hooper as the person
shakes their hips (ask your teacher what a hula-hoop is). But the
shifting of a planet's orbit is much slower than for the hula hoop. It
takes many, many centuries for the Earth's orbit to shift entirely
around once. These changes would be slow and small. It is an active
area of research whether or not the orbiting objects in the solar
system can indefinitely orbit the Sun without ever undergoing
drastic changes in their orbits --- they may actually change dramatically at some point: such changes would be called "chaotic" and
the area of research is called "chaos theory". No new forces would be
necessary to make this possibility happen; it would simply be the
application of gravity once again. There are known examples of chaotic
behavior in the solar system, but only involving a few small objects
orbiting the outer planets, or in the asteroid belt. Chaotic behavior
is defined as happening when very small differences in the initial or
current conditions of an experiment, or of the solar system's motions
(perhaps so small as to not be easily measured), would lead to
drastically different results later in time.

Besides the planets in the solar system, it is even possible (over a
much longer time scale) for a passing neighbor star to cause small
changes in the planets' orbits. That's actually how comets, which
otherwise have large orbits that never take them near the Sun, are
caused to change orbit and pass near the Sun, allowing us to see them.
If the incoming comet passes near Jupiter, it may be permanently moved
into a small orbit that will keep it repeatedly passing near the Sun
(once every 75 years, or so, for Halley's comet, for example). An
example of chaos: in this case, if Jupiter is in just a slightly
different position when the comet passes during its first fall toward
the Sun, the comet may not end up in a small orbit due to Jupiter's
gravitational pull, but may instead end up hitting the Sun! Or it
might be "ejected" from the solar system entirely.